1. Field of the Invention
The present invention relates generally to a zoom lens system using a diffractive optical element (hereinafter DOE for short), and more specifically to a zoom lens system used for a phototaking optical system for silver-salt cameras, electronic cameras, etc.
2. Discussion of Related Art
Many types of compact cameras with built-in zoom lenses are now commercially available and have become widespread due to user friendliness. Recent zoom lenses having ever higher zoom ratios are also growing in popularity. However, cameras with such zoom lenses incorporated therein are poor in regard to portability because of their increased size. For this reason, there is a strong demand for a compact yet high-power zoom lens.
As the zoom ratio of a zoom lens becomes high, some problems arise in conjunction with correction of aberrations. Generally, a zoom lens is constructed of a plurality of moving lens groups to vary the focal length by varying spacings between them. However, the heights of light rays through each lens group vary upon zooming, resulting in a variation in the amount of aberrations produced. For this reason, even when the remnant aberrations at each zoom lens group is canceled and corrected by other zoom lens groups, it is impossible to maintain good performance over the entire zooming spaces because aberration fluctuations are still produced upon zooming. In particular, a high-power zoom lens is susceptible to large aberration fluctuations. To reduce such aberration fluctuations, it is thus required that sufficient correction of aberrations be made at individual zoom lens groups. In the prior art, this has been achieved by increasing the number of lenses in each lens group.
However, this is unfavorable for achieving size reductions of the lens system because the length of the lens system becomes physically long. In recent years, many proposals have been made to construct a zoom lens using an aspherical surface or surfaces with no increase in the number of lenses involved. So far, two- and three-group types have been widely used for zoom lenses. However, the two-group type is superior to the three-group type using an increased number of lenses with a complicated lens barrel.
Therefore, if an aspherical lens is properly used with the two-group type, then spherical aberration and coma can be effectively corrected with no increase in the number of lenses, and so high zoom ratio and compactness can be achieved at the same time. This is already disclosed in JP-A 8-338946. In the zoom lens disclosed there, the first and second lens groups are each constructed of two lenses in spite of a zoom ratio of 1.9 to 2.9. Chromatic aberrations are corrected by proper determination of the Abbe's numbers of the lenses.
For users, on the other hand, zoom lenses available at an ever-more reasonable price are demanded.
The cost of a certain lens system may be cut down by decreasing the number of lenses involved, as mentioned above. However, such cost reductions may also be achieved by constructing lenses of plastics. Plastic materials are cheaper, and can be more easily processed into aspherical lenses at a much lower cost, as compared with glass materials. Thus, plastics is very favorable in view of cost-effectiveness. However, a grave problem with plastics is that its refractive index or shape properties are susceptible to changes depending on environmental changes such as temperature or humidity changes. For instance, when a lens having power is constructed of plastics, a shift of the focus position is likely to occur depending on environmental changes, resulting in a drop in performance. At a telephoto end of a zoom lens in particular, this shift becomes large enough to have more of an adverse influence on performance.
To solve such problems thereby achieving both low cost and high performance, the applicant has come up with a zoom lens system using a powerless lens, as typically disclosed in JP-A 5-113537. In this case, the zoom ratio achieved is of the order of 1.5 to 2.2.
On the other hand, attention is recently focused on a diffractive optical element (DOE) which can make use of diffraction to bend light rays. The DOE is different from a general vitreous material in that it has an Abbe's number of -3.45 or reciprocal dispersion, and has a characteristic feature of enabling achromatization using a positive power and positive power combination, unlike the prior art refractive systems.
Some exemplary applications of such a diffractive optical element to a zoom lens are disclosed in JP-A's 9-197273 and 9-197274. JP-A 9-197273 shows a two-group type zoom lens comprising a first lens group consisting of one positive lens and a second lens group consisting of one negative lens, and using a diffractive surface. The zoom ratio is of the order of 1.6 to 1.9. JP-A 9-197274 discloses a two-group type zoom lens consisting of four lenses and using a diffractive surface, as in the aforesaid JP-A 8-338946. Examples 2 and 5 teach that a zoom ratio of as high as about 3.4 is achievable.
However, these prior art zoom lenses have such problems as mentioned below.
The zoom lens system set forth in JP-A 8-338946 is constructed of four lenses. As the number of lenses decreases, however, it is impossible to make sufficient correction for chromatic aberrations because the aspherical surface itself makes no contribution to correction of the chromatic aberrations. Even given the proper determination of the Abbe's numbers of the lenses, no sufficient performance can be obtained due to the limitation of correction.
JP-A 9-197274 shows the correction of chromatic aberrations by using a diffractive optical element in a similar arrangement. However, the zoom lens disclosed fails to take full advantage of the diffractive optical element to make sufficient correction for chromatic aberration, partly because of an imbalance between longitudinal chromatic aberration and chromatic aberration of magnification, and partly because of increased secondary spectra. Thus, this zoom lens is still less than satisfactory in terms of performance although it has high power.
For a zoom lens system comprising a reduced number of lenses such as one set forth in JP-A 8-338946 or JP-A 9-197274, it is required to increase the powers of a positive lens and a negative lens so that aberrations are corrected while the power of each lens group is kept. This results in an increased sensitivity to fabrication errors. For instance, when there is an error in the curvature of each surface in the first lens group or in the spacing between lenses in the second lens group, a very large focus position shift occurs at the telephoto end. It is thus required to place fabrication accuracy under severe control. However, this leads to another problem in connection with lens processing or lens assembly.
JP-A 5-113537 shows the use of a powerless plastic lens thereby achieving cost reductions. However, an arrangement using a powerless lens has a design defect that chromatic aberrations cannot be corrected. Further, this arrangement is applicable only to a zoom lens system having relatively low power; that is, it has a very narrow application, because only one lens having power is used in the second lens group, and so some chromatic aberrations remain theoretically.
JP-A 9-197273 shows that each lens group is constructed of one lens and remnant chromatic aberrations are corrected with a diffractive optical element. Correction of spherical aberration and coma remains still insufficient; that is, the comprehensive performance is still less than satisfactory. The arrangement disclosed uses a glass lens having a very large aspherical amount, and so fails to achieve significant cost reductions. While this arrangement uses a plastic material lens, no care is taken of a drop in performance depending on environmental changes such as temperature or humidity changes. Thus, many practical problems remain unsolved.
JP-A 9-197274 shows that each lens group is constructed of two lenses, and chromatic aberrations are corrected with a diffractive optical element. However, the cost problem remains unsolved because of using a glass aspherical surface. Although the third positive lens is made up of plastics, a performance problem associated with environmental changes remains unsolved because that lens has strong power.